Various microwave imaging systems have been proposed to satisfy the demand for improved security inspection systems, such as those used in airports to screen passengers and baggage. At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors (hereinafter referred to as “antenna elements”) to capture either passive microwave radiation emitted by a target associated with the person or other object or reflected microwave radiation reflected from the target in response to active microwave illumination of the target. A two-dimensional or three-dimensional image of the person or other object is constructed by scanning the array of antenna elements with respect to the target's position and/or adjusting the frequency (or wavelength) of the microwave radiation being transmitted or detected.
Microwave imaging systems typically include transmit, receive and/or reflect antenna arrays for transmitting, receiving and/or reflecting microwave radiation to/from the object. Microwave radiation is generally defined as electromagnetic radiation having wavelengths between radio waves and infrared waves. Such antenna arrays can be constructed using traditional analog phased arrays or binary reflector arrays. In either case, the antenna array typically directs a beam of microwave radiation containing a number of individual microwave rays towards a point or area/volume in 3D space corresponding to a voxel or a plurality of voxels in an image of the object, referred to herein as a target. This is accomplished by programming each of the antenna elements in the array with a respective phase shift that allows the antenna element to modify the phase of a respective one of the microwave rays. The phase shift of each antenna element is selected to cause all of the individual microwave rays from each of the antenna elements to arrive at the target substantially in-phase. The resulting microwave image of the object can be displayed as a two-dimensional (2D) or three-dimensional (3D) image to an operator. Examples of programmable antenna arrays are described in U.S. patent application Ser. Nos. 10/997,422, entitled “A Device for Reflecting Electromagnetic Radiation,” and 10/997,583, entitled “Broadband Binary Phased Antenna.”
In traditional phased arrays, the custom is to place the antenna elements apart by λ/2 in both directions to suppress sidelobes throughout a hemispherical scan. The number of antenna elements in a circular area array is about π(D/λ)2 where D is the diameter of the circle and A is the wavelength of the radiation. The number of antenna elements, and therefore the cost of the array, is proportional to (D/λ)2. Each antenna element has traditionally been controlled by its own active device. However, the active devices used in controlling the antenna elements can be expensive, and in some cases may even require one or more stages of amplifiers. Even when the active devices are relatively inexpensive, the system may require a very deep digital memory to support a large set of focal areas or volumes.
One approach for reducing the number of antenna elements is to simply omit elements from the traditional “dense” phased array. The result is known as a “sparse array”. While using a sparse array does reduce the number of active devices required, a new problem is created. Sparse arrays are well-known in the ultrasound and microwave/millimeter-wave literature to be associated with grating sidelobes. Sidelobes produce unwanted ghosting phenomena in the scanning or imaging process.
Various remedies have been tried to remove or negate the effect of the sidelobes. For example, deconvolution algorithms can be applied but the most successful of these are nonlinear algorithms which are both scene-dependent and very time-consuming. Two of the most popular deconvolution algorithms are CLEAN and the Maximum Entropy Method or MEM. An older, linear (and hence faster and more general) algorithm is Wiener-Helstrom filtering, but it is well known that it produces inferior image reconstruction compared to nonlinear (slower, more specialized) techniques such as Maximum Likelihood (ML) iteration. Correlation imaging, involving different subsets of an already sparse array, is another nonlinear scheme which tends to be quite slow. In some cases, e.g., radioastronomy, one has prior knowledge about the scene (say, from visible telescopes) which can be used to weed out much of the ghost phenomena. However, this solution is inadequate whenever one is dealing with a highly dynamic environment.
U.S. Application for patent Ser. No. 11/552,193, entitled “Convex Mount for Element Reduction in Phased Arrays with Restricted Scan,” which was filed on Oct. 20, 2006, and U.S. Application for patent Ser. No. 11/551,382, entitled “Element Reduction in Phased Arrays with Cladding,” which was filed on Oct. 20, 2006, disclose that when the range of solid scan angle is less than 2π steradians (i.e., less than a hemisphere), it is theoretically possible to reduce the element count without sidelobe degradation. However, U.S. Application for patent Ser. No. 11/552,193 requires that the antenna elements be mounted on a curved surface, and U.S. Application for patent Ser. No. 11/551,382 requires a special material to be applied to the surface of the antenna elements.
Therefore, a need still remains for a reduced-device phased array on a flat surface that does not suffer from sidelobe degradation.